Cell size influences inorganic carbon acquisition in artificially selected phytoplankton

Strain-specific metabarcoding reveals rapid evolution of copper tolerance in populations of the coastal diatom Skeletonema marinoi

Phytoplankton have short generation times, flexible reproduction strategies, large population sizes and high standing genetic diversity, traits that should facilitate rapid evolution under directional selection. We quantified local adaptation of copper tolerance in a population of the diatom Skeletonema marinoi from a mining-exposed inlet in the Baltic Sea and in a non-exposed population 100 km away. We hypothesized that mining pollution has driven evolution of elevated copper tolerance in the impacted population of S. marinoi. Assays of 58 strains originating from sediment resting stages revealed no difference in the average tolerance to copper between the two populations. However, variation within populations was greater at the mining site, with three strains displaying hyper-tolerant phenotypes. In an artificial evolution experiment, we used a novel intraspecific metabarcoding locus to track selection and quantify fitness of all 58 strains during co-cultivation in one control and one toxic copper treatment. As expected, the hyper-tolerant strains enabled rapid evolution of copper tolerance in the mining-exposed population through selection on available strain diversity. Within 42 days, in each experimental replicate a single strain dominated (30%-99% abundance) but different strains dominated the different treatments. The reference population developed tolerance beyond expectations primarily due to slowly developing plastic response in one strain, suggesting that different modes of copper tolerance are present in the two populations. Our findings provide novel empirical evidence that standing genetic diversity of phytoplankton resting stage allows populations to evolve rapidly (20-50 generations) and flexibly on timescales relevant for seasonal bloom progressions.

Disentangling top-down drivers of mortality underlying diel population dynamics of Prochlorococcus in the North Pacific Subtropical Gyre

Photosynthesis fuels primary production at the base of marine food webs. Yet, in many surface ocean ecosystems, diel-driven primary production is tightly coupled to daily loss. This tight coupling raises the question: which top-down drivers predominate in maintaining persistently stable picocyanobacterial populations over longer time scales? Motivated by high-frequency surface water measurements taken in the North Pacific Subtropical Gyre (NPSG), we developed multitrophic models to investigate bottom-up and top-down mechanisms underlying the balanced control of Prochlorococcus populations. We find that incorporating photosynthetic growth with viral- and predator-induced mortality is sufficient to recapitulate daily oscillations of Prochlorococcus abundances with baseline community abundances. In doing so, we infer that grazers in this environment function as the predominant top-down factor despite high standing viral particle densities. The model-data fits also reveal the ecological relevance of light-dependent viral traits and non-canonical factors to cellular loss. Finally, we leverage sensitivity analyses to demonstrate how variation in life history traits across distinct oceanic contexts, including variation in viral adsorption and grazer clearance rates, can transform the quantitative and even qualitative importance of top-down controls in shaping Prochlorococcus population dynamics.

The requirement for external carbonic anhydrase in diatoms is influenced by the supply and demand for dissolved inorganic carbon

Photosynthesis by marine diatoms contributes significantly to the global carbon cycle. Due to the low concentration of CO2 in seawater, many diatoms use extracellular carbonic anhydrase (eCA) to enhance the supply of CO2 to the cell surface. While much research has investigated how the requirement for eCA is influenced by changes in CO2 availability, little is known about how eCA contributes to CO2 supply following changes in the demand for carbon. We therefore examined how changes in photosynthetic rate influence the requirement for eCA in three centric diatoms. Modeling of cell surface carbonate chemistry indicated that diffusive CO2 supply to the cell surface was greatly reduced in large diatoms at higher photosynthetic rates. Laboratory experiments demonstrated a trend of an increasing requirement for eCA with increasing photosynthetic rate that was most pronounced in the larger species, supporting the findings of the cellular modeling. Microelectrode measurements of cell surface pH and O2 demonstrated that individual cells exhibited an increased contribution of eCA to photosynthesis at higher irradiances. Our data demonstrate that changes in carbon demand strongly influence the requirement for eCA in diatoms. Cell size and photosynthetic rate will therefore be key determinants of the mode of dissolved inorganic carbon uptake.

Phenotypic Analysis and Molecular Characterization of Enlarged Cell Size Mutant in Nannochloropsis oceanica

The cell cycle is the fundamental cellular process of eukaryotes. Although cell-cycle-related genes have been identified in microalgae, their cell cycle progression differs from species to species. Cell enlargement in microalgae is an essential biological trait. At the same time, there are various causes of cell enlargement, such as environmental factors, especially gene mutations. In this study, we first determined the phenotypic and biochemical characteristics of a previously obtained enlarged-cell-size mutant of Nannochloropsis oceanica, which was designated ECS. Whole-genome sequencing analysis of the insertion sites of ECS indicated that the insertion fragment is integrated inside the 5'-UTR of U/P-type cyclin CYCU;1 and significantly decreases the gene expression of this cyclin. In addition, the transcriptome showed that CYCU;1 is a highly expressed cyclin. Furthermore, cell cycle analysis and RT-qPCR of cell-cycle-related genes showed that ECS maintains a high proportion of 4C cells and a low proportion of 1C cells, and the expression level of CYCU;1 in wild-type (WT) cells is significantly increased at the end of the light phase and the beginning of the dark phase. This means that CYCU;1 is involved in cell division in the dark phase. Our results explain the reason for the larger ECS size. Mutation of CYCU;1 leads to the failure of ECS to fully complete cell division in the dark phase, resulting in an enlargement of the cell size and a decrease in cell density, which is helpful to understand the function of CYCU;1 in the Nannochloropsis cell cycle.

Phytoplankton community shift in response to experimental Cu addition at the elevated CO2 levels (Arabian Sea, winter monsoon)

Understanding phytoplankton community shifts under multiple stressors is becoming increasingly important. Among other combinations of stressors, the impact of trace metal toxicity on marine phytoplankton under the ocean acidification scenario is an important aspect to address. Such multiple stressor studies are rare from the Arabian Sea, one of the highest productive oceanic provinces within the North Indian Ocean. We studied the interactive impacts of copper (Cu) and CO₂ enrichment on two natural phytoplankton communities from the eastern and central Arabian Sea. Low dissolved silicate (DSi < 2 µM) favoured smaller diatoms (e.g. Nitzschia sp.) and non-diatom (Phaeocystis). CO₂ enrichment caused both positive (Nitzschia sp. and Phaeocystis sp.) and negative (Cylindrotheca closterium, Navicula sp., Pseudo-nitzschia sp., Alexandrium sp., and Gymnodinium sp.) growth impacts. The addition of Cu under the ambient CO₂ level (A-CO₂) hindered cell division in most of the species, whereas Chla contents were nearly unaffected. Interestingly, CO₂ enrichment seemed to alleviate Cu toxicity in some species (Nitzschia sp., Cylindrotheca closterium, Guinardia flaccida, and Phaeocystis) and increased their growth rates. This could be related to the cellular Cu demand and energy budget at elevated CO₂ levels. Dinoflagellates were more sensitive to Cu supply compared to diatoms and prymnesiophytes and could be related to the unavailability of prey. Such community shifts in response to the projected ocean acidification, oligotrophy, and Cu pollution may impact trophic transfer and carbon cycling in this region.

Phytoplankton growth and community shift over a short-term high-CO2 simulation experiment from the southwestern shelf of India, Eastern Arabian Sea (summer monsoon)

The southwestern shelf water of India (eastern Arabian Sea) experiences high seasonality. This area is one of the understudied regions in terms of phytoplankton response to the projected ocean acidification, particularly, during the summer monsoon when phytoplankton abundance is high. Here we present the results of a short-term simulated ocean acidification experiment (ambient CO₂ 424 µatm; high CO₂, 843, 1138 µatm) on the natural phytoplankton assemblages conducted onboard (R. V. Sindhu Sadhana) during the summer monsoon (Aug 2017). Among the dissolved inorganic nutrients, dissolved silicate (DSi) and nitrate + nitrite levels were quite low (< 2 µM). Phytoplankton biomass did not show any net enhancement after the incubation in any treatment. Both marker pigment analysis and microscopy revealed the dominance of diatoms in the phytoplankton community, and a significant restructuring was noticed over the experimental period. Divinyl chlorophylla (DVChla) containing picocyanobacteria and 19'-hexanoyloxyfucoxanthin (19'HF) containing prymnesiophytes did not show any noticeable change in response to CO₂ enrichment. A CO₂-induced positive growth response was noticed in some diatoms (Guinardia flaccida, Cylindrotheca closterium, and Pseudo-nitzschia sp.) and dinoflagellates (Protoperidinium sp. and Peridinium sp.) indicating their efficiency to quickly acclimatize at elevated CO₂ levels. This is important to note that the positive growth response of toxigenic pennate diatoms like Pseudo-nitzschia as well as a few dinoflagellates at elevated CO₂ levels can be expected in the future-ocean scenario. The proliferation of such non-palatable phytoplankton may impact grazing, the food chain, and carbon cycling in this region.

Colimitation assessment of phytoplankton growth using a resource use efficiency approach in the Bay of Seine (French-English Channel)

Eutrophication and dystrophy are two of the main problems affecting coastal ecosystems. In the Bay of Seine, phosphorus (P) inputs from the Seine estuary have been largely reduced in the last decade, in contrast to nitrogen (N), which leads to high N/P ratio inputs. To study the effect of dystrophy, an enrichment bioassay using water sampled from the Bay of Seine was repeated 19 times over a period of 18 months with six different enrichments. After a few days, chlorophyll a (chl a), alkaline phosphatase activity (APA), transparent exopolymeric particles (TEPs), cytometric size structure, and maximum quantum yield of photosystem II were measured. The data provide strong evidence for an N & P colimitation system in the vast majority of the incubations, as only the N + P and N + P + Si enrichments supported phytoplankton growth, and Si only appeared to play a secondary role in our incubations. A N/P ratio of 16 equal to the Redfield ratio was identified as the optimum for balanced growth, as chl a was the highest and TEP and APA production was the lowest at this ratio. To fit the requirements of the colimited system, a new resource use efficiency (RUE_NP) calculation was developed to account for N and P colimitation instead of only one nutrient, as is usually the case. This calculation allows better representation of RUE in dystrophic conditions, as found in many highly anthropized ecosystems. The relationships between RUE_NP and the size structure of the phytoplankton community were explored, and a significant positive correlation between RUE_NP and larger cells (>2 μm) and a negative correlation with smaller cells (<2 μm) were noted, showing a better use of nutrients by larger cells. This study highlights an increase of RUE_NP with the phytoplankton cell size in a colimited system.

Distinctive correlations between cell concentration and cell size to microalgae biomass under increasing carbon dioxide

Carbon capture and storage (CCS) via microalgae cultivations is getting renewed interest as climate change mitigation effort, owing to its excellent photosynthetic and CO₂ fixation capability. Microalgae growth is monitored based on their biomass, cell concentrations and cell sizes. The key parametric relationships on microalgae growth under CO₂ are absent in previous studies and this inadequacy hampers the design and scale-up of microalgae-based CCS. In this study, three representative microalgae species, Chlorella, Nostoc and Chlamydomonas, were investigated for establishing key correlations of cell concentrations and sizes towards their biomass fluctuations under CO₂ influences of 0% to 20% volume ratios (v/v). This revealed that Chlorella and Chlamydomonas cell concentrations significantly contributed towards increasing biomass concentration under CO₂ elevations. Chlorella and Nostoc cell sizes were enhanced at 20% (v/v). These findings provided new perspectives on growth responses under increasing CO₂ treatment, opening new avenues on CCS schemes engineering designs and biochemical production.